Method of making thin-film magnetic recording medium having perpendicular anisotropy

ABSTRACT

A magnetic recording medium having a magnetizable metallic thin-film layer of perpendicular anisotropy can be produced at commercially viable production rates by electrodepositing cobalt, hypophosphite, and preferably also nickel ions at a plating bath temperature of 50°-80° C. and a plating current density of at least 20 mA/cm 2 .

TECHNICAL FIELD

The invention concerns a method of making a metallic thin-film magneticrecording medium having perpendicular anisotropy, i.e., an easy-axis ofmagnetization perpendicular to the plane of the thin-film layer.

BACKGROUND ART

While virtually all magnetic recording media now in use havemagnetizable layers comprising magnetizable particles dispersed inorganic binder, the amount of information that can be recorded in anysuch medium is reaching the theoretical limit. It has long been knownthat information can be recorded more compactly on metallic thin-filmmagnetic recording media such as the medium disclosed in U.S. Pat. No.2,644,787 (Bonn et al.). That patent disclosed a method ofelectrodepositing onto an electrically conductive substrate amagnetizable layer from an aqueous plating bath having a pH in the rangeof 2 to 6 and including as essential elements nickel ions in aconcentration in the range of 0.2 to 1.7N, cobalt ions in the range of0.2 to 1.0N and hypophosphite ions in the range of 0.04 to 0.2N. Currentdensities ranged from 10 to 200 amperes per square foot (11 to 215mA/cm²) and bath temperatures were on the order of 45 to 55° C. with anupper limit of about 80° C. Higher temperatures may have producedspontaneous reduction throughout the bath. A specific product wasreported to have a coercivity of approximately 810 oersteds and aremanence of the order of 10,000 gauss from a bath containing 0.84N innickel, 0.84N in cobalt, 0.145N in hypophosphite ions, and ammoniumchloride in 1.9 molar concentration, with a current density of 50amperes per square foot (54 mA/cm²) at 50° C.

U.S. Pat. No. 3,578,571 (McQuaid et al.) disclosed an effort to reducethe cost of thin-film magnetic recording media by reducing the highconcentration of components in the plating bath. Improved coercivitylevels and particularly squareness ratios were said to be achieved,although the reported coercivities were within the range of the Bonnpatent. McQuaid's bath temperatures were between 32° and 66° C., andcurrent densities were between 5 and 10 amps/dm² (50 and 100 mA/cm²).

It can be assumed that the magnetic values of the Bonn and McQuaid mediawere measured in the planes of the coatings. Media made according to theexample of each of those patents would exhibit mixed in-plane andperpendicular magnetic anisotropy.

Sato et al., "Magnetic Properties of Electro-deposited Co-Ni-P RecordingFilms", Denki Kagaku, Vol. 35, No. 2, pp. 111-15, 1967, (a publicationin the Japanese language), also concerns electrodeposition ofmagnetizable thin-film coatings from a bath of cobalt, nickel andhypophosphite ions. The Sato publication reports that when the currentdensity was kept low (0.5 A/dm² or 5 mA/cm²), the magnetizable coatinghad perpendicular anisotropy. Somewhat higher current densities resultedin a distributed orientation, and at a current density of 2.0 A/dm² or20 mA/cm², the coating had in-plane anisotropy. The only bathtemperature mentioned was 25° C. The pH was 6.0.

Because the crystallite c-axes of electro-deposited coatings havingin-plane anisotropy tend to be oriented in all planar directions, it isrecognized that perpendicular anisotropy should enable more compact andefficient recording of information, since all crystallite c-axes can beoriented in the same direction. Also, magnetic recording media havingperpendicular anisotropy exhibit a significantly reduced demagnetizingeffect at high recording densities.

Recent efforts to develop thin-film magnetic recording media havingperpendicular anisotropy have concentrated on cobalt-chromium coatings.See, for example, U.S. Pat. No. 4,210,946 (Iwasaki). No recentpublications have been found concerning cobalt-nickel-hypophosphiteelectrodeposition, perhaps because of Sato's teaching of very lowcurrent densities to obtain perpendicular anisotropy, hence economicallyimpractical rates of electrodeposition.

DISCLOSURE OF INVENTION

The present invention is an improvement over Sato by teaching conditionsfor making thin-film magnetic recording media having perpendicularanisotropy by electrodeposition at relatively high current densities,thereby providing economically viable rates of deposition. Like Sato'smethod, the method of the present invention involves electrodepositingonto an electrically conductive substrate a magnetizable coating from anaqueous plating bath having a pH between 2 and 6 and including at leastcobalt and hypophosphite ions. Although always used by Sato, nickel ionsare not important in the present invention unless high saturationmagnetization is important. The cobalt and hypophosphite ionconcentrations useful in practicing the present invention are comparableto those taught by Sato but are preferably within the ranges of 0.2 to1.0N, and 0.1 to 0.4N, respectively, and the weight ratio of Co to P isbetween 5:1 and 50:1. Also comparable to Sato's teachings, nickel ionsmay be present up to about 1.5N as long as the ratio of Ni to Co doesnot exceed about 3 to 2. Also as in Sato, the aqueous bath may include abuffering agent such as ammonium chloride within a range of 1.0 to 3.0N.

The method of the present invention differs from that of Sato bymaintaining a bath temperature and a plating current density of at least20 mA/cm² within Region I of FIG. 2 of the drawing while keeping thetemperature below a level that would produce excessive autocatalyticdeposition.

Like many thin-film layers, the magnetizable thin-film layer of thepresent invention may have a thin, tough overcoating to protect it fromcontact with magnetic recording heads. Such a protective coating isespecially useful when the recording medium is a flexible disk or tape,but less needed when the recording medium is a rigid disk.

Unless the substrate of the recording medium has anelectrically-conductive surface, it is necessary to apply anelectrically-conductive underlayer such as copper or permalloy, an alloyof about 80% nickel and 20% iron. Permalloy affords the added advantageof providing an efficient magnetic return path when using the medium forperpendicular recording. Because permalloy is only marginallyelectrically-conductive, it preferably is applied over a layer ofcopper. Both copper and permalloy may be conveniently applied by vapordeposition or sputtering.

When the substrate is a plastic film, the thickness of theelectrically-conductive underlayer should be sufficient to preventburning from resistive heating at the cathodic contacts to the film. Onemicrometer was sufficient for a moving 75-micrometer polyester film.When the substrate is an aluminum disk, an electrically-conductiveunderlayer may be desirable because aluminum is difficult to plate dueto its rapid surface oxide formation.

THE DRAWING

In the drawing:

FIG. 1 is a semi-log plot of coercivity measured perpendicularly as afunction of current density for metallic thin-film magnetic recordingmedia of the invention, which media were obtained by electrodepositionof preferred concentrations of cobalt, nickel and hypophosphite ions ata plating bath temperature of 65° C.;

FIG. 2 indicates the magnetic anisotropies of thin-film media of thepresent invention obtained by electrodeposition at various plating bathtemperatures and current densities;

FIG. 3 is a photomicrograph at 300,000× magnification of an in-planesection of the magnetic recording layer of a thin-film magneticrecording medium of the invention; and

FIG. 4 is a photomicrograph at 300,000× magnification of a perpendicularsection through the magnetic recording layer of another thin-filmmagnetic recording medium of the invention.

FIG. 1 is a plot 10 of magnetometer measurements of coercivitiesperpendicular to the magnetic recording layers of the metallic thin-filmmagnetic recording media of Examples 1 and 2 which wereelectro-deposited from aqueous plating baths at 65° C. at variouscurrent densities. When deposited at current densities below about 135mA/cm², as indicated by a dashed line 12 at the approximate edge ofRegion I, the recording layers should have perpendicular anisotropy.When deposited at current densities above about 230 mA/cm², as indicatedby a dashed line 14 at the approximate edge of Region III, the recordinglayers should have in-plane anisotropy. Within the intermediate RegionII, the deposit displays a mixture of in-plane and perpendicularanisotropy.

A decrease in the plating bath temperature has the same effect as anincrease in current density. This effect is illustrated in FIG. 2 whichshows the same three approximate Regions I, II and III of perpendicular,mixed, and in-plane anisotropy, respectively obtained at various bathtemperatures and current densities as reported in Example 3. Noappreciable variations in the boundaries of those regions have resultedfrom changes in concentrations within the ranges of 0.2 to 1.0N cobalt,0 to 1.5N nickel and 0.1 to 0.4N hypophosphite ions.

Within Region I, a bright, shining, continuous, overall deposit ofcobalt, nickel and phosphorous (hereinafter called "CoNiP") is obtained.In Region A beyond Region I, less than overall coverage may be obtained.In Region B beyond Region III, the deposit tends to have a black andburnt appearance, apparently due to microscopic irregularities, possiblystemming from hydroxides and other impurities in the deposit.

Since substantial autocatalytic deposition may occur at plating bathtemperatures above about 80° C., the presently preferred temperature isabout 75° C. in order to provide economical production rates withvirtually no autocatalytic waste. At 75° C. and 240 mA/cm², a depositionrate of about 70 nm/sec is obtained. At 50° C. and 35 mA/cm², thedeposition rate is only about 12 nm/sec. Lower deposition rates may becommercially impractical.

As indicated in FIG. 1, a coercivity of 2200 oersteds is obtainable ateconomical production rates. A CoNiP magnetic recording medium of theinvention having that coercivity exhibited a saturation magnetizationexceeding 12000 gauss. These values are higher than can be obtained withcobalt-chromium media which are presently the best known thin-film mediaexhibiting perpendicular anisotropy.

To obtain the photomicrograph of FIG. 3, the medium of Example 4 wascarefully washed in acetone to remove its backside coating, after whichthe metallic thin-film deposit was masked with an identical coating. Theexposed copper side was electrothinned using a 36% solution oforthophosphoric acid in water and a current density of 500 mA/cm².During thinning a jet of the solution was directed at the center of thesample to cause faster thinning near the center. When sufficient copperwas removed to expose roughly a 3 mm diameter area of CoNiP deposit, theelectrothinning was stopped and the masking coating was carefullyremoved with acetone. The sample was then electrothinned from both sidesusing the traditional window technique with a solution of 6% perchloricacid in methanol cooled in a dry-ice acetone bath.

The sample was mounted between two 75 mesh (3mm) grids and photographedat 100,000× using a JEM-200cx transmission electron microscope. Electrondiffraction patterns show the grains to have their crystallite c-axesperpendicular to the sample surface.

The photomicrograph of FIG. 4 was obtained from the double-faced CoNiPmedium of Example 5 by microtoming. These photomicrographs reveal thecolumnar nature of the grains. The columns are roughly 20-30 nm indiameter and mostly extend through the entire thickness of the deposit.The striations lying parallel to the surface within each column are dueto stacking faults in the crystallite planes normal to the c-axis.

Energy dispersive X-ray studies using a JEM-200cx scanning transmissionelectron microscope suggest phosphorous has a higher concentration atthe grain boundaries which, possibly in combination with hydroxides orother impurities, has a decoupling effect such that the individualgrains act as single-domain areas. Phase contrast studies using thetransmission electron microscope also suggest that the grain boundarieshave a lower atomic density. The grain size obtained at the edge ofRegion I near Region II is on the order of 20 nm, and at the oppositeedge of Region I near Region A is about 100 nm. Since smaller grainsizes provide higher coercivities, one obtains media of desiredcoercivities by selecting various areas of Region I.

From the above-reported and other studies it has been determined thatmagnetic recording layers of media of the invention have hexagonal,close-packed crystal structure. Electron diffraction studies indicatecrystallite clusters about one micrometer in width, and the a-axes ofall crystallites in each cluster lie in the plane of the plated area andin the same direction within about 3 or 4 degrees. The a-axes ofcrystallites of each cluster adjacent to that cluster lie in the sameplane and are usually shifted about 30 degrees.

EXAMPLE 1

A piece of clean copper sheet was masked with pressure-sensitiveadhesive tape to cover one face and to expose two areas on the otherface, one 1.3 by 1.3 cm and the other 1.3 by 2.5 cm. An aqueous platingbath was prepared as follows: 56 g CoSO₄.7H₂ O, 37 g NiSO₄.6H₂ O, 19 gNaH₂ PO₂.H₂ O, and 75g NH₄ Cl were measured into a 1-liter beaker andwater was added to make 700 ml of solution. These chemicals were A.C.S.grade. The resulting solution had the following normalities: 0.57NCoSO₄.7H₂ O, 0.4N NiSO₄.6H₂ O, 0.25N NaH₂ PO₂.H₂ O, and 2.0N NH₄ Cl.

While heated to 65° C. in a 600 ml Pyrex beaker, 500 ml of the solutionwas slowly stirred, and the larger unmasked area of the piece of copperwas immersed in the plating bath while electrical contact to the otherunmasked area was made outside of the bath. Facing the immersed area ata distance of about 6 cm was a cobalt anode. The copper was plated for30 seconds at 520 mA total current, giving an average current density of160 mA/cm². The true current density at the center of the plated area ofthe resulting CoNiP metallic thin-film magnetic recording medium wasdetermined to be 115 mA/cm². The thickness of the magnetizable thin-filmlayer at the center of the plated area was about 1.2 micrometers. Thecomposition of the thin-film layer, found by X-ray fluorescencemeasurements, was 82.5% cobalt, 13.6% nickel and 3.9% phosporus byweight. X-ray and electron diffraction studies showed that allcrystallite c-axes had a preferred orientation normal to the plane ofthe thin-film layer.

A magnetometer curve of magnetic moment versus applied field wasdetermined at the center of the plated area. The coercivity measuredperpendicular to the magnetometer field and the plated area was 2250 Oeand the saturation moment was 12,000 gauss. The shape of the curveindicated good perpendicular anisotropy.

EXAMPLE 2

A number of metallic thin-film magnetic recording specimens wereprepared as in Example 1, except at 20 different current densities,while also adjusting the plating time to keep the deposit thicknessesequal. Magnetometer measurements provided data used to generate the plot10 of FIG. 1.

EXAMPLE 3

A number of metallic thin-film magnetic recording specimens wereprepared as in Example 1, except at various bath temperatures, whileadjusting the plating times to keep the deposit thicknesses equal.Magnetometer measurements were used to generate FIG. 2 of the drawing.

EXAMPLE 4

A metallic thin-film magnetic recording medium was made as in Example 1except at a bath temperature of 60° C. and an average current density of60 mA/cm². Also, the piece of copper sheet was 1 cm square, had athickness of 0.025 mm, and was completely unmasked on the face side,while the backside was masked by an acetone-soluble coating. Plating wascompleted in one minute to provide a CoNiP thin-film layer about 1.0micrometer in thickness. This medium, which also had perpendicularanisotropy, was used in preparing the photomicrograph of FIG. 3 of thedrawing.

The true current density at the center of the plated area was determinedto be 45 mA/cm². The magnetometer curve of a sample taken from thecenter of the plated area showed the sample to have perpendicularanisotropy and a coercivity of 1400 Oe with a saturation magnetizationof 11,900 gauss measured perpendicularly to the magnetometer field andthe plated area.

EXAMPLE 5

A metallic thin-film magnetic recording medium was made as in Example 4except the copper sheet was unmasked and thus plated on both sides ofits one square cm area. The thickness of the thin-film layer was about0.5 micrometer. This medium was used to provide FIG. 4 of the drawing.

EXAMPLE 6

A metallic thin-film magnetic recording medium was prepared as inExample 1 except as follows:

Unmasked plated area: 4 by 3.2 cm

Plating bath:

0.46N CoSO₄.7H₂ O

0.46N NiSO₄.6H₂ O

0.24N NaH₂ PO₂.H₂ O

1.9N NH₄ Cl

pH 4.3

Bath temperature: 55.7° C.

Plating time: 520 seconds

Average current density: 69 mA/cm²

A circular disk 3 mm in diameter was punched from the medium at aposition 0.8 cm from a short edge and 1.6 cm from a long edge. The diskwas measured in the perpendicular direction in a vibrating samplemagnetometer. Magnetic moment measurements indicated a thickness of 11micrometers, and from comparisons of the thicknesses at other positionson the disk, the true current density used in plating the disk wasdetermined to have been 60 mA/cm². Its coercivity was 2200 oersteds, andits saturation magnetization was 12,200 gauss. The shape of themagnetization versus applied field curve indicated that the sample hadgood perpendicular anisotropy. X-ray fluorescence showed 82.1% cobalt,16.1% nickel and 1.9% phosphorous by weight.

I claim:
 1. Method of making a magnetic recording medium comprisingelectrodepositing onto an electrically conductive substrate amagnetizable layer from an aqueous plating bath having a pH between 2and 6 and including at least cobalt and hypophosphite ions inconcentrations within the range of 0.2 to 1.0 N and 0.1 to 0.4 N,respectively, the weight ratio of Co to P in the magnetizable layerbeing between 5:1 and 50:1, wherein the improvement comprisesmaintaining a bath temperature and a plating current density withinRegion I of FIG. 2 of the drawing while maintaining a plating currentdensity of at least 20 mA/cm² and a bath temperature between about 60°and 80° C., under which conditions the deposited layer has predominantlyperpendicular magnetic anisotropy.
 2. Method as defined in claim 1wherein the bath includes nickel ions from about 0.4N to 1.5N inconcentration, and the weight ratio of Ni to Co does not exceed about 3to
 2. 3. Method as defined in claim 1 wherein the current density is atleast 100 mA/cm².
 4. Method as defined in claim 1 wherein the bathincludes a buffering agent within the range of 1.0 to 3.0N.
 5. Method asdefined in claim 4 wherein the buffering agent is ammonium chloride.